Methods of reducing vibrations for electric motors

ABSTRACT

A method of controlling an electric motor includes pulsing the electric motor and phase shifting the modulation frequency. Pulsing the electric motor at the modulation frequency propels a vehicle to increase efficiency of the electric motor. Phase shifting the modulation frequency includes phase shifting between 0 degrees and 180 degrees to reduce vibrations induced in the vehicle.

BACKGROUND 1. Technical Field

The present disclosure relates to methods of reducing vibrations forelectric motors, and more specifically, to reducing vibration of pulsedelectric motors by phase shifting the pulses of the electric motors.

2. Discussion of Related Art

Electric motors are known to be efficient at providing continuous torqueto driven equipment. The torque delivery of electric motors is typicallycontinuous without the pulsations associated with an internal combustionengine. Generally, electric motors have an optimal efficiency point inmid-low to mid-high range of torque relative to a maximum torque of theelectric motor. For example, the maximum efficiency of an electric motormay be in a range of 30% to 80% of the maximum torque of the electricmotor.

When an electric motor provides a continuous torque in a low range ofthe maximum torque of the electric motor, e.g., below 20% of the maximumtorque, the efficiency of the electric motor is typically low. It hasbeen found that reducing a duty cycle of the electric motor by pulsingthe electric motor at the optimal efficiency point can provide a targettorque in a low range of the electric motor at a higher efficiency thanproviding a continuous torque from the electric motor. The pulsing ofthe electric motor at the optimal efficiency point includes deliveringpulses at a modulation frequency.

The pulsing of the electric motor at a modulation frequency can inducevibrations in equipment driven by the electric motor. For example, whenthe electric motor is driving a vehicle, the pulsing of the electricmotor can create vibrations in the structure of the vehicle. Thesevibrations can be amplified when the modulation frequency is near anatural frequency resonance of the vehicle structure.

SUMMARY

This disclosure relates generally to methods of modifying a modulationfrequency of an electric motor to reduce or cancel vibrations resultingfrom pulsing of the electric motor. The method may include phaseshifting and/or time shifting the modulation frequency to reduce orcancel vibrations resulting from pulsing of the electric motor.

In an embodiment of the present disclosure, a method of controlling anelectric motor includes pulsing the electric motor and phase shiftingthe modulation frequency. Pulsing the electric motor at the modulationfrequency reduces a duty cycle of the electric motor to increaseefficiency of the electric motor. Phase shifting the modulationfrequency includes phase shifting between 0 degrees and 180 degrees toreduce vibrations induced in the driven equipment.

In embodiments, the phase shifting occurs when the modulation frequencyis within a resonance range of the driven equipment. The resonance rangemay be defined within 10 Hertz (Hz) of a resonance frequency of thedriven equipment.

In some embodiments, pulsing the electric motor includes pulsing theelectric motor at a pulse torque to deliver a target torque less thanthe pulse torque. Pulsing the electric motor at the pulse torque mayinclude the pulse torque being an optimal efficiency point of theelectric motor.

In certain embodiments, the method includes varying the modulationfrequency to generate a target torque. Phase shifting the modulationfrequency occurs when the modulation frequency is below 100 Hz. Phaseshifting may include phase shifting the modulation frequency at ashifting frequency. Phase shifting the modulation frequency may includethe shifting frequency being greater than the modulation frequency.

In particular embodiments, the method may include time shifting peaks ofthe phase shifted modulation frequency to level torque delivery of theelectric motor. Pulsing the electric motor at the modulation frequencymay propel a vehicle such that the driven equipment is a drive shaft ofthe vehicle.

In another embodiment of the present disclosure, a non-transitorycomputer-readable medium has instructions stored thereon that, whenexecuted by a controller, causes the controller to pulse an electricmotor at a modulation frequency to reduce a duty cycle of the electricmotor to deliver a target torque and increase efficiency of the electricmotor and phase shift the modulation frequency between 0 degrees (0radians) and 180 degrees (π radians) to reduce vibrations resulting frompulsing the electric motor.

In embodiments, the phase shifting occurs when the modulation frequencyis within a resonance range of a structure to which the electric motoris mounted or a component driven by the electric motor. Pulsing theelectric motor may include pulsing the electric motor at a pulse torqueto deliver a target torque less than the pulse torque. Pulsing theelectric motor at the pulse torque may include the pulse torque being anoptimal efficiency point of the electric motor.

In some embodiments, the instructions further cause the controller tovary the duty cycle to generate a target torque. Phase shifting themodulation frequency may occur when the modulation frequency is below100 Hz.

In certain embodiments, phase shifting includes phase shifting themodulation frequency at a shifting frequency. The shifting frequency maybe greater than the modulation frequency.

In particular embodiments, the instructions further cause the controllerto time shift peaks of the phase shifted modulation frequency to leveltorque delivery of the electric motor.

In another embodiment of the present disclosure, a controller to operatean electric motor to rotate a driven component includes a processor anda memory including a program to cause the processor to pulse theelectric motor at a modulation frequency to reduce a duty cycle and toincrease efficiency of the electric motor to deliver a target torque tothe driven component and phase shrift the modulation frequency between 0degrees (0 radians) and 180 degrees (π radians) at a shifting frequency.

In embodiments, the phase shifting occurs when the modulation frequencyis within a resonance range, the resonance range may be stored in thememory of the controller. Pulsing the electric motor may include pulsingthe electric motor at a pulse torque to deliver a target torque that isless than the pulse torque.

In some embodiments, the program may further cause the processor to varythe modulation frequency to generate a target torque. Phase shifting mayinclude phase shifting the modulation frequency at a shifting frequency.The program may further cause the processor to time shift peaks of thephase shifted modulation frequency to level torque delivery of theelectric motor.

In another embodiment of the present disclosure, a drive systemdiscloses a structure, a driven component, an electric motor fixed tothe structure for rotating the driven component, and a controller. Thecontroller configured to operate the electric motor to rotate the drivencomponent. The controller including a processor and a memory including aprogram. The program causes the processor to pulse the electric motor ata modulation frequency to deliver a target torque to the drivencomponent and to phase shift the modulation frequency between 0 degrees(0 radians) and 180 degrees (π radians) at a shifting frequency toreduce vibrations within the structure.

Further, to the extent consistent, any of the embodiments or aspectsdescribed herein may be used in conjunction with any or all of the otherembodiments or aspects described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present disclosure are described hereinbelow withreference to the drawings, which are incorporated in and constitute apart of this specification, wherein:

FIG. 1 is a schematic view of an electric motor mounted to a structureof a vehicle to model a response to the vibrations of the torquedelivery of the electric motor;

FIG. 2 is a chart showing an exemplary frequency response of thestructure of the vehicle of FIG. 1 ;

FIG. 3 is a graph of a torsional response of the vehicle of FIG. 1 inview of pulsed torque delivery of the electric motor at a naturalresonant frequency of the structure of the vehicle with and without avibration cancellation feature provided in accordance with an embodimentof the present disclosure at the natural resonant frequency of thestructure of the vehicle of FIG. 1 ;

FIG. 4 is a graph of the torsional response of the vehicle of FIG. 1 inview of pulsed torque delivery of the electric motor at a naturalresonant frequency of the structure of the vehicle with and without avibration cancellation feature provided in accordance with an embodimentof the present disclosure across a range of frequencies;

FIG. 5 is a chart of a phase modified control signal provided inaccordance with an embodiment of the present disclosure imposed over anunmodified control signal to pulse an electric motor; and

FIG. 6 is a flow chart of a method of controlling an electric motorprovided in accordance with an embodiment of present disclosure.

DETAILED DESCRIPTION

The present disclosure will now be described more fully hereinafter withreference to example embodiments thereof with reference to the drawingsin which like reference numerals designate identical or correspondingelements in each of the several views. These example embodiments aredescribed so that this disclosure will be thorough and complete, andwill fully convey the scope of the disclosure to those skilled in theart. Features from one embodiment or aspect can be combined withfeatures from any other embodiment or aspect in any appropriatecombination. For example, any individual or collective features ofmethod aspects or embodiments can be applied to apparatus, product, orcomponent aspects or embodiments and vice versa. The disclosure may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. As used in the specification and the appended claims, thesingular forms “a,” “an,” “the,” and the like include plural referentsunless the context clearly dictates otherwise. In addition, whilereference may be made herein to quantitative measures, values, geometricrelationships or the like, unless otherwise stated, any one or more ifnot all of these may be absolute or approximate to account foracceptable variations that may occur, such as those due to manufacturingor engineering tolerances or the like.

To increase efficiencies of an electric motor in a low torque range ofthe electric motor, the electric motor may be pulsed to reduce a dutycycle of the electric motor to provide a target torque or demand torqueas an average torque delivered over time by pulsing the electric motorat an optimal efficiency point at a modulation frequency. This pulsingof the electric motor may have a Pulse Width Modulation (PWM) waveformfor torque delivery. The duty cycle is selected to provide a low targettorque to the driven equipment while pulsing the electric motor at theoptimal efficiency point. The modulation frequency may be selected tosatisfy noise, vibration, and harshness (NVH) requirements and to reduceor minimize transition losses. In certain embodiments, the modulationfrequency is selected based on a torsional vibration of the drivenequipment For example, an electric motor may be pulsed at an efficienttorque of 200 Nm with a 20% duty cycle to prove a target torque of 40 Nmto driven equipment. Depending on the NVH characteristics of the drivenequipment, the 200 Nm pulses may be delivered at a 30 Hz modulationfrequency. In an exemplary electric motor, in certain operatingcondition, pulsing the electric motor to lower a duty cycle to deliverthe target torque has been shown to be 9% more efficient than theelectric motor providing torque demanded through continuous torquedelivery.

Generally, electric motors provide a substantially continuous torque. Asa result, electric motors typically may be directly mounted to structureand are directly coupled to driven equipment. This is different frominternal combustion motors that are typically mounted to structure byone or more vibration isolating mounts to reduce the transfer ofvibrations from the motor to the structure. Similarly, internalcombustion motors may include vibration isolating elements, e.g., aflywheel, such that the pulsations in torque delivery from the internalcombustion motor are isolated from being transferred to the drivenequipment. As a result of being directly mounted to structure and thedriven equipment, pulsing an electric motor at a modulation frequencymay result in undesirable vibrations being transmitted to structureand/or driven equipment. In particular, the torsional vibrations as aresult of pulsing the electric motor may result in undesirablevibrations in structure and/or driven equipment. In some embodiments,electric motors may be mounted with compliant mounts that isolate somevibration from the electric motor.

With reference to FIGS. 1-4 , the result of a modulation frequency beingat a natural resonant frequency of the structure of a vehicle aremodeled. FIG. 1 shows a simplified model of a vehicle 10 being driven bya pulsed electric motor 20. FIG. 2 shows an exemplary model of afrequency response of the vehicle 10 over a range of frequenciesincluding two natural resonant frequencies 32, 34. The naturalresonances of the structure of the vehicle 10 have peaks or naturalresonant frequencies at 17 Hz and 77 Hz.

FIGS. 3 and 4 show the modeled torsional response of the vehicle 10 whenthe electric motor 20 is pulsed at one of the natural resonantfrequencies, e.g., 77 Hz. The torsional response of the vehicle 10 issignificant and matches the modulation frequency of the electric motor20. Also shown in FIG. 4 , the torsional response of the vehicle 10 issubstantially isolated to the modulation frequency of 77 Hz.

When an electric motor, e.g., electric motor 20, is used to propel ordrive a vehicle 10, peaks in a torsional response of the vehicle 10 mayresult in undesirable vibrations being felt by passengers of the vehicle10. The undesirable vibrations may also cause premature wear or failureof components of the vehicle 10. For example, undesirable vibrations incomponents of the drive train may result in premature wear and/orfailure of these components. As such, it is desirable to reduce theamplitude of or eliminate the undesirable vibrations of the vehicle 10and/or the drivetrain.

As detailed above, pulsing the electric motor 20 at an optimalefficiency point at a modulation frequency to reduce the duty cycle ofthe electric motor 20 allows for the delivery of a target torque belowthe optimal efficiency point at a higher efficiency than continuouslyproviding the target torque from the electric motor 20. The low targettorque may be in a range of 0 percent to 40 percent of the optimalefficiency point of the electric motor 20. The target torque deliveredby the electric motor 20 can be controlled by increasing or decreasingthe duty cycle of an excitation torque at which the electric motor 20 ispulsed or excited. The excitation torque may be selected to be anoptimal efficiency point for the electric motor 20 and may be in a rangeof 50 percent to 80 percent, e.g., 60 percent, of the maximum torque ofthe electric motor 20.

With the excitation torque fixed at an optimal efficient point of theelectric motor 20, the torque delivered by the electric motor 20 can becontrolled by adjusting the duty cycle of the electric motor 20. Forexample, the duty cycle can be increased to increase the torquedelivered and the duty cycle can be decreased to lower the torquedelivered. The modulation frequency can be increased or decreased as theduty cycle changes based on NVH characteristics of the driven equipment.In some embodiments, a lower modulation frequency may reduce transitionlosses of the electric motor 20 as the electric motor 20 is pulsed.

The electric motor 20 may provide a continuous torque when the targettorque is near the optimal efficiency point of the electric motor 20,for example, when the target torque is within 20% of the optimalefficiency point. When the target torque is more than 20% below theoptimal efficiency point, a controller of the electric motor 20 mayreduce the duty cycle of the electric motor 20 by pulsing the electricmotor 20 at the optimal efficiency point to provide the target torque.The controller may reduce the duty cycle to decrease the torquedelivered to the target torque.

Referring now to FIGS. 3-5 , a method of canceling vibrations frompulsing an electric motor is detailed in accordance with the presentdisclosure. The method of canceling vibration can be executed in acontroller of the electric motor 20 without the need for vibrationmitigation hardware, e.g., vibration isolating engine mounts or a flywheel. The method includes phase shifting the modulation frequency at ashifting frequency such that vibration induced by pulsing the electricmotor 20 at its modulation frequency is reduced or completely canceled.The shifting frequency may interact with the modulation frequency of theelectric motor 20 and introduce lower frequency content so that theselection of the shifting frequency is done in such a way that theoverall torsional vibration response of the driven equipment isminimized compared to steady phase pulsation while operating within thelimitations of the inverter and maintaining the efficiency gains frompulsing the electric motor 20. In the examples below, the modulationfrequency is phase shifted between 0 degrees (0 radians) and 180 degrees(π radians) at the shifting frequency. In some embodiments, themodulation frequency may be phase shifted between different phases or bephase shifted between more than two frequencies. To phase shift themodulation frequency, the controller of the electric motor can controlthe timing of the current switching of the electric motor. The method ofcanceling vibrations may be active whenever the controller pulses theelectric motor or may only be active when pulsing the electric motor 20would result in unacceptable NVH of the driven equipment.

With particular reference to FIG. 5 , a phase shifted modulation signal140 is shown relative to the original modulation signal 130 with themodulation frequency being 77 Hz and the shifting frequency being 100Hz. As shown, the phase shifted modulation signal 140 provides similartorque to the original modulation signal 130. The phase shiftedmodulation signal 140 has some peaks 155, 149 that align with the peaks132, 134 of the original modulation signal 130 and some peaks 158 thatare in direct opposition from the peaks 132 of the original modulationsignal 130. In addition to matching or opposing the peaks of theoriginal modulation signal 130, the phase shifted modulation signal hasadditional peaks, e.g., peaks 142, 144, 146, 148, 152, 154, 158, thatare between peaks of the original modulation signal 130.

With additional reference back to FIGS. 3 and 4 , phase shifting theoriginal modulation signal 130 as represented by the phase shiftedmodulation signal 140 such that even at a resonant frequency of thevehicle, vibrations induced by pulsing the electric motor 20 aresignificantly reduced when compared to the original modulation signal130. As shown in FIG. 3 , the torsional response 50 of the vehicle 10 tothe phase shifted modulation signal 140 is between one quarter to onethird the amplitude or magnitude when compared the torsional response 40of the vehicle 10 to the original modulation signal 130. Also apparentis the reduction in amplitude or magnitude of the peak torsionalresponse 50 to the phase shifted modulation signal 140 of FIG. 4 byapproximately 75% from the peak of the torsional response 40 of thevehicle to the original modulation signal 130.

With particular reference to FIG. 5 , the phase shifted modulationsignal 140 may have lulls or plateaus that result in uneven torquedelivery. This uneven torque delivery may result in a noticeable jerk orlag in torque delivery such that the torque delivery is outsidedesirable NVH characteristics. The method may include time shiftingportions of the phase shifting to even out the torque delivery to makethe torque deliver more consistent to eliminate any noticeable jerks orlags in the torque delivery.

As shown in FIG. 5 , the phase delay signal is a sinusoidal wave. Itsome embodiments, the phase shifting may have a trapezoidal form such asa PWM waveform with a transition ramp. The frequency of the phase delayf_(p) may be modeled by the following equation:

$f_{p} = {\frac{1}{{2a} + \frac{1}{r}}f_{m}}$

where the frequency of the phase delay f_(p) is based on the originalmodulation frequency f_(m) with the number of cycles between phaseshifts represented as “a” and the phase shift transition rate beingrepresented by “r”. While a sinusoidal and PWM waveform are describedherein, other waveforms are contemplated without deviating from thescope of this disclosure.

Referring now to FIG. 6 , a method of controlling an electric motor isdisclosed in accordance with the present disclosure and is referred togenerally as method 200. The method 200 is executed on a controller thatprovides signals to an electric motor to deliver a target torque to adrive component. The method 200 is described in accordance with themodel of an electric motor 20 and vehicle 10 of FIG. 1 . However, thedrive component may be a driveshaft or axle of a vehicle or may be adriveshaft to rotate a piece of equipment.

The method 200 may include a controller of the electric motor 20receiving an input signal requesting a target torque from the electricmotor 20 (Step 210). The controller analyzes the requested target torqueto determine if the target torque is within a continuous operation rangeof the electric motor 20 (Step 220). The continuous operation range maybe a range of torques that are at or above the optimal efficiency pointof the electric motor 20. The continuous operation range may include arange of torques that are below the optimal efficiency point of theelectric motor 20. For example, when the optimal efficiency point of theelectric motor 20 is 60% of the maximum torque of the electric motor 20,the continuous operation range may be from 40% to 100% of the maximumtorque of the electric motor 20.

When the requested target torque is within continuous operation range,the controller operates the electric motor 20 to deliver the targettorque as a continuous torque (Step 230).

When the requested target torque is below the continuous operationrange, the controller selects a modulation frequency to pulse theelectric motor 20 to deliver the target torque (Step 240). As detailedabove, the modulation frequency is selected such that the electric motor20 can be pulsed at the optimal efficiency point to deliver the targettorque. The duty cycle is adjusted to set the torque delivered from theelectric motor 20 to the target torque while pulsing the electric motor20 at the modulation frequency. For example, to increase a torquedelivered from the electric motor 20, the duty cycle is increased and todecrease a torque delivered from the electric motor 20, the duty cycleis decreased. As the duty cycle is increased or decreased, themodulation frequency may be increased or decreased based on the NVHcharacteristics of the driven equipment, e.g., the vehicle.

With the modulation frequency selected, the controller analyzes themodulation frequency in view of the resonance frequencies of thestructure that the electric motor 20 is mounted to and the resonancefrequencies of the driven equipment, e.g., a structure of a vehicleand/or driven components of the vehicle, to determine if the modulationfrequency is in a resonance range (Step 250). The resonance range may beone or more range of frequencies at or near resonances of the structuresupporting the electric motor 20 or equipment driven by the electricmotor 20. The resonance range may be any frequency or may be defined aswithin a predefined frequency of the resonances of the structure ordriven equipment. Using the example above with resonances at 17 Hz and77 Hz the resonance range may be within 10 Hz of the resonances suchthat the resonance range is 7 Hz to 27 Hz and 67 Hz to 87 Hz. In someinstances, the resonance range may vary with the frequency of theresonance. For example, using the same resonances of 17 Hz and 77 Hz,the resonance range may be 7 Hz to 27 Hz and 57 Hz to 97 Hz. Theresonance range may vary as a percentage of each of the resonantfrequencies. In some embodiments, the resonance range may be defined aswhen the modulation frequency is below a threshold frequency. Forexample, the threshold frequency of the resonance range may be 100 Hzsuch that when the modulation frequency is at or below 100 Hz, themodulation frequency is modified or phase shifted. When the modulationfrequency is outside of the resonance range, the controller may operatethe electric motor 20 at the modulation frequency without modifying themodulation frequency (Step 260).

When the modulation frequency is within the resonance range, thecontroller operates the electric motor 20 at a modified or phase shiftedmodulation frequency (Step 280). To modify the modulation frequency, thecontroller activates a vibration control program or algorithm to phaseshift the modulation frequency (Step 270). The vibration control programor algorithm includes the controller adjusting the timing of the currentswitching of the electric motor 20 to phase shift the modulationfrequency at a shifting frequency. As noted above, the shiftingfrequency may be greater than the modulation frequency, may be less thanthe modulation frequency, or may be the same as the modulationfrequency. In some embodiments, the shifting frequency may be constantor may change as the modulation frequency changes. The phase shifting ofthe modulation frequency may decrease an amplitude of or preventvibrations in the structure or driven equipment as a result of pulsingthe electric motor 20 as shown in FIG. 5 . In certain embodiments, thephase shifting the modulation frequency may create an uneven torquedelivery in the phase shifted modulation frequency. The controller mayinclude a time shifting algorithm that time shifts peaks of the phaseshifted modulation frequency to deliver torque more consistently andeliminate any noticeable jerks of lags in the torque delivery (Step275).

The controller detailed above may be a standalone controller or may bepart of another controller. The controller includes a processor and amemory. The controller may also include an input to receive input suchas a desired torque. The controller includes a motor output that is insignal communication with an electric motor to operate the electricmotor to provide a target torque. The methods detailed above may bestored in the memory of the controller as a non-transitorycomputer-readable medium that when executed on the processor of thecontroller cause the controller to execute the methods detailed aboveincluding method 200.

While several embodiments of the disclosure have been shown in thedrawings, it is not intended that the disclosure be limited thereto, asit is intended that the disclosure be as broad in scope as the art willallow and that the specification be read likewise. Any combination ofthe above embodiments is also envisioned and is within the scope of theappended claims. Therefore, the above description should not beconstrued as limiting, but merely as exemplifications of particularembodiments. Those skilled in the art will envision other modificationswithin the scope of the claims appended hereto.

What is claimed:
 1. A method of controlling an electric motor, themethod comprising: pulsing the electric motor at a modulation frequencyto reduce a duty cycle of the electric motor and to increase efficiencyof the electric motor; and phase shifting the modulation frequencybetween 0 degrees (0 radians) and 180 degrees (π radians) to reducevibrations induced in driven equipment.
 2. The method according to claim1, wherein the phase shifting occurs when the modulation frequency iswithin a resonance range of the driven equipment.
 3. The methodaccording to claim 2, wherein the resonance range is defined within 10Hertz (Hz) of a resonance frequency of the driven equipment.
 4. Themethod according to claim 1, further comprising varying the modulationfrequency to generate a target torque.
 5. The method according to claim1, wherein phase shifting the modulation frequency occurs when themodulation frequency is below 100 Hz.
 6. The method according to claim1, wherein phase shifting includes phase shifting the modulationfrequency at a shifting frequency.
 7. The method according to claim 6,wherein phase shifting the modulation frequency includes the shiftingfrequency being greater than the modulation frequency.
 8. The methodaccording to claim 1, further comprising time shifting peaks of thephase shifted modulation frequency to level torque delivery.
 9. Anon-transitory computer-readable medium having instructions storedthereon that, when executed by a controller, cause the controller to:pulse an electric motor at a modulation frequency to reduce a duty cycleof the electric motor to deliver a target torque and increase efficiencyof the electric motor; and phase shift the modulation frequency between0 degrees (0 radians) and 180 degrees (π radians) to reduce vibrationsresulting from pulsing the electric motor.
 10. The non-transitorycomputer-readable storage medium according to claim 9, wherein the phaseshifting occurs when the modulation frequency is within a resonancerange of a structure to which the electric motor is mounted or acomponent driven by the electric motor.
 11. The non-transitorycomputer-readable storage medium according to claim 9, wherein phaseshifting the modulation frequency occurs when the modulation frequencyis below 100 Hz.
 12. The non-transitory computer-readable storage mediumaccording to claim 9, wherein phase shifting includes phase shifting themodulation frequency at a shifting frequency.
 13. The non-transitorycomputer-readable storage medium according to claim 12, wherein phaseshifting the modulation frequency includes the shifting frequency beinggreater than the modulation frequency.
 14. The non-transitorycomputer-readable storage medium according to claim 9, wherein theinstructions further cause the controller to time shift peaks of thephase shifted modulation frequency to level torque delivery.
 15. Acontroller to operate an electric motor to rotate a driven component,the controller comprising: a processor; and a memory including a programto cause the processor to: pulse the electric motor at a modulationfrequency to reduce a duty cycle and increase efficiency of the electricmotor to deliver a target torque to the driven component; and phaseshift the modulation frequency between 0 degrees (0 radians) and 180degrees (π radians) at a shifting frequency.
 16. The controlleraccording to claim 15, wherein the phase shifting occurs wherein themodulation frequency is within a resonance range, the resonance rangebeing stored in the memory of the controller.
 17. The controlleraccording to claim 15, wherein the program further causes the processorto vary the modulation frequency to generate a target torque.
 18. Thecontroller according to claim 15, wherein phase shifting includes phaseshifting the modulation frequency at a shifting frequency.
 19. Thecontroller according to claim 15, wherein the program further causes theprocessor to time shift peaks of the phase shifted modulation frequencyto level torque delivery.
 20. A drive system comprising: a structurehaving at least one resonant frequency; a driven component; an electricmotor fixed to the structure for rotating the driven component; and acontroller according to claim 15.